Cutoff Frequency Calculator

The cutoff frequency, also called the -3dB frequency, is the point at which the output signal power drops to half of its maximum value, corresponding to a voltage drop to approximately 70.7% of the input. For low-pass filters, frequencies below this point pass through; for high-pass filters, frequencies above it pass through. See also our Resonant Frequency Calculator (LC).

For an RC filter, the cutoff frequency is calculated as f = 1 / (2π × R × C), where R is resistance in ohms and C is capacitance in farads. The time constant τ = R × C, and ω = 1/τ is the angular cutoff frequency in radians per second.

For an RL filter, the cutoff frequency is f = R / (2π × L), where R is resistance in ohms and L is inductance in henrys. The time constant is τ = L / R. Both RC and RL filters are first-order filters with a -20 dB/decade roll-off beyond the cutoff.

LC filters use the resonant frequency formula f = 1 / (2π × √(L × C)), where L is inductance in henrys and C is capacitance in farads. Unlike RC and RL filters, an ideal LC filter has no resistive losses and produces a sharper response at the resonant frequency. You might also find our Inductance — Coil Inductance useful.

A low-pass filter passes signals with frequencies below the cutoff frequency and attenuates those above it — useful for smoothing signals or removing high-frequency noise. A high-pass filter does the opposite: it blocks low-frequency signals and passes those above the cutoff, commonly used to remove DC offsets or low-frequency hum.

Enter resistance in ohms (Ω), capacitance in farads (F), and inductance in henrys (H). For smaller values, convert first: 1 µF = 0.000001 F, 1 nF = 0.000000001 F, 1 mH = 0.001 H, 1 µH = 0.000001 H. The calculator uses SI base units throughout.

A passive filter uses only passive components — resistors, capacitors, and inductors — with no external power supply or amplification. An active filter incorporates active components like op-amps that can amplify the signal. Passive filters are simpler and more reliable but cannot boost signal levels; active filters offer more flexibility and sharper roll-off characteristics.

This calculator solves for the cutoff frequency given known component values. To find a component value from a known frequency, rearrange the formula: for RC, C = 1/(2π × f × R) or R = 1/(2π × f × C); for RL, L = R/(2π × f); for LC, L = 1/(4π² × f² × C).